It is apparent that in future new battery systems which have to meet very stringent requirements in terms of reliability, safety, efficiency and service life will be used both for stationary applications, such as wind power plants, in motor vehicles which are configured as hybrid or electric motor vehicles, and in electronic devices such as laptops or mobile phones.
In vehicles with an at least partially electric drive, batteries are used to store the electrical energy for the electric motor which assists the drive or serves as a drive. In vehicles of the latest generation, for example so-called lithium-ion battery cells are used in this context. Said cells are characterized, inter alia, by high energy densities and extremely low self-discharging. Lithium-ion battery cells have a positive and a negative electrode at which lithium ions can reversibly migrate in (intercalation) or migrate out again (deintercalation). As a rule, a plurality of battery cells are combined to form a battery module, and then a plurality of battery modules are combined to form a battery by parallel or series connection. An important requirement here is an effective battery management system which monitors the function of the individual cells of the battery and controls the charging process thereof. In this way, for example defective cells can be switched off and/or bypassed and state messages relating to the state of charge can be output.
Lithium-ion battery cells according to the prior art have, as a rule, a metallic cell housing in which an electrode winding is arranged. The electrode winding comprises two metallic substrates which are coated with the active cathode material or anode material. Between the two substrates there is a separator. The cell is connected electrically via the upper side and underside of the electrode winding.
In addition it is known that lithium-ion battery cells are subjected to considerable expansion of volume and contraction of volume due to the different states of charge; that is to say the electrode windings of the cells expand during charging and contract again during discharging. These changes in volume are caused by the processes of lithium ions migrating in and out in the electrode. Given corresponding deformation of the cell, this change in volume would be extended further outward beyond the casing of the battery cell and would lead to a change in the geometric dimensions of the battery. For this reason, in lithium-ion battery cells according to the prior art the housing is composed of a rigid metallic body. DE 10 2009 028 986 A1 discloses a method and a device by means of which a premature wear or ageing process of a lithium-ion battery, due to the expansion of the volume owing to the state of charge of the battery cells of the lithium-ion battery, can be prevented and a sufficiently low internal resistance can be ensured in a simple manner. For this purpose, a counter-pressure is applied to the lithium-ion battery by the device as a function of the volume of said battery. However, to do this the pressure originating from the battery cells has to be determined very precisely as a function of the battery volume.
A disadvantage with the solutions according to the prior art is, inter alia, that the positioning of a pressure sensor in the wall of the housing or on the outside of the housing cannot provide any direct information about the change in volume of the electrode winding. Furthermore, at present no prior-art methods or devices are known by means of which the state of health or the precise state of charge can be inferred directly from the pressure originating from the battery cells of a lithium-ion battery or the force which constitutes the pressure.